1. Field of the Invention
The present invention generally relates to a cordless telephone system. More specifically, the present invention is directed to a cordless telephone system equipped with a detachable radio unit capable of achieving an outdoor telephone communication.
2. Description of the Prior Art
Recently, a so-called "cordless telephone system" (namely, a system wherein wireless communication is available between a base unit (telephone receiver unit) and a remote mobile unit (cordless telephone handset) in a telephone set) has been widely utilized. That is, a base unit connected to a public telephone network has one or plural mobile units, and a radio telephone communication is available from this mobile unit via the base unit to an arbitrary telephone subscriber.
In the known cordless telephone system, wireless (radio) communications are available between one mobile unit and the base unit, and also between one mobile unit and the other mobile unit. However, since such a radio communication must be carried out via the base unit, there is a certain limitation in the communication route. That is, although the radio communication is available between two mobile units inside a housing, no radio communication is available between them outside the housing (outdoor radio communication), but also between the base unit installed inside the housing and the outdoor mobile unit.
Very recently, a so-termed "second generation cordless telephone system" has been proposed in which a private radio communication station is installed either in a housing, or a firm, and also a public radio communication station is installed outside the housing or firm, whereby a telephone communication can be established by using the home-use cordless telephone receivers (i.e., mobile units) not only within the home or firm, but also outside the home or firm. Also in the second generation cordless telephone system, the radio power outputs of the base unit and the mobile units are limited to those of the conventional cordless telephone system. Under such a circumstance, practically speaking, the radio communication is available only between the base unit and the mobile unit.